Adsorption processes have been effectively utilized for a number of years to perform both bulk separations and purification separations in a variety of technology areas, e.g., hydrocarbon processing, hydrogen purification, air separation and pollution control. Despite the diversity of processes and particular applications, most adsorption processes are influenced by mass transfer limitations to one extent or another.
When a feed mixture is passed through an adsorber bed containing an activated adsorbent, i.e., one having adsorption capacity for at least one component present in the feed mixture, at suitable adsorption conditions, a mass transfer zone is established and advances through the adsorber bed as more feed is passed through the adsorbed bed. The term "mass transfer zone" is generally accepted in the art and denotes that section of the adsorber bed which is undergoing dynamic changes in both adsorbent loading of the adsorbate, i.e., component being adsorbed, and concentration of the adsorbate in the feed mixture. That is, at the leading edge of the mass transfer zone, and ahead of it, the adsorbate concentration is reduced relative to the feed and is substantially in equilibrium with the activated adsorbent, and at the trailing edge of the mass transfer zone, and behind it, the adsorbate concentration is substantially equal to that in the feed mixture and the adsorbent is substantially loaded to capacity with the adsorbate. The portion of the adsorbed bed behind the mass transfer zone is generally known as the equilibrium zone. See, for example, the discussion of mass transfer zone concepts and related adsorption topics in the publications: Lukchis, Adsorption Systems, Part I-Design by Mass-Transfer-Zone Concept, Chemical Engineering, June 11, 1973 at pp. 111-116; Lukchis, Adsorption Systems, Part II-Equipment Design, Chemical Engineering, July 9, 1973 at pp. 83-87; Lukchis, Adsorption Systems, Part III-Adsorbent Regeneration, Chemical Engineering, Aug. 6, 1973 at pp. 83-90.
The size, or length, of the mass transfer zone can be dependent on various factors well known in the art including the type of adsorbent, the particle size of the adsorbent, the gas velocity, the temperature of the feed mixture and the adsorbent and the concentration of the adsorbate. When the rate-controlling step in the adsorption process is diffusion of the adsorbate into the adsorbent particle, the particle size of the adsorbent can have a substantial effect on the size of the mass transfer zone. It is generally known by those skilled in the art that in such diffusion limited processes, adsorbents having smaller particle sizes can provide smaller mass transfer zones and hence, more efficient utilization of the adsorber bed. That is, when the mass transfer zone is smaller, i.e., more compact, more adsorbate can be loaded into the adsorbent before the leading edge of the mass transfer zone breaks through the effluent end of the adsorber bed. Hence, higher product purities and recoveries can be achieved and, therefore, the use of smaller particles is generally desirable.
However, everything else being equal, smaller particles cause higher pressure drop. Note, for example the Ergun equation which sets forth the relationship of pressure drop in a fixed bed: ##EQU1## where .DELTA.P is the pressure drop across a bed of depth L; M and P are the viscosity and density of the mixture, respectively; G is the superficial mass flow rate; .epsilon. is the void fraction; and D.sub.p is the effective particle diameter and is defined as D.sub.p =6V.sub.p /A.sub.p where V.sub.p and A.sub.p are the volume and external surface area of a single particle. See page 29, Section 4, R. H. Perry, C. H. Chilton, Chemical Engineer's Handbook, McGraw-Hill Book Company, New York, fifth edition, 1973. Higher pressure drops, in turn, cause increased lifting and crushing forces on the adsorbent bed as gases are passed therethrough. Since larger particles generally have a higher crush strength and can tolerate higher pressure drops, the choice of adsorbent particle size is often made on the basis of the pressure drop through the adsorber bed. The following adsorption processes describe the use of particulate adsorbents having various particle sizes.
U.S. Pat. No. 3,359,198 discloses methods for the treatment of recycle gas streams from continuous processing units by contact with a solid bed of adsorbent to remove undesirable components from the recycle gas. In one aspect of the invention, an enriched hydrogen purity recycle gas is produced by passing a recycle gas stream containing undesirable components through an adsorber bed containing adsorbent preferably comprising solid particles of a size range of from about 10 mesh to about 60 mesh and removing the purified product therefrom. The above-identified patent discloses that this size range is preferable since it will result in a significant pressure drop in the direction of flow through the fixed bed thereby insuring efficient contact of the gas with solid adsorbent and preventing channeling and bypassing of a portion of the bed.
U.S. Pat. No. 3,564,816 discloses a pressure swing adsorption process (PSA) for separating gas mixtures having selectively adsorbable components, as for example, CO, CO.sub.2, CH.sub.4 and other light saturated or unsaturated hydrocarbons, NH.sub.3, H.sub.2 S, Ar, N.sub.2 and H.sub.2 O from hydrogen, and O.sub.2, N.sub.2 and CO.sub.2 from air. Examples 1 and 2 of this patent, respectively, disclose the use of adsorber beds containing 1/16" calcium Zeolite A molecular sieve pellets to separate nitrogen from air and to separate nitrogen from admixture with hydrogen. In processes, such as described in the above-identified patent, adsorbent particles of about the size disclosed are often used in order to prevent damage to the particles due to excessive pressure drop and to provide a relatively constant pressure profile throughout the adsorber bed during the pressure changing steps, particularly during desorption steps when high residual pressures in portions of the adsorber bed can adversely effect the desorption of the adsorbate.
U.S. Pat. No. 4,176,053 discloses the separation of normal paraffins from a mixed paraffin feedstock by selective adsorption on crystalline zeolitic molecular sieves. One aspect of the patent provides a constant-pressure process wherein a non-adsorbable purge gas, e.g., hydrogen, is used to desorb the normal paraffins from a 1/16" calcium Zeolite A adsorbent.
U.S. Pat. No. 4,194,892 discloses a rapid adiabatic pressure swing process with a total cycle time of less than 30 seconds using a single adsorbent bed of No. 20.times.No. 120 mesh particles. In processes utilizing rapid pressure swing adsorption (RPSA), high pressure drops are generally desired and the use of small adsorbent particles provides the necessary flow resistance to operate the process. However, an excessive pressure drop even in RPSA can be disadvantageous. For example, as set forth at col. 9, lines 6, et seq., of the above-identified patent, an adsorber bed having too much resistance can have reduced product recovery. This patent discloses that the problem of reduced product recovery caused by excessive pressure drop can be solved by increasing the adsorbent particle size.
U.S. Pat. No. 4,608,061 discloses a PSA process for separating normal butane from mixtures thereof with isobutane by a particular sequence of countercurrent depressurization and purge steps. One aspect of the invention disclosed in the patent provides a pressure swing adsorption process for separating normal and isobutane using 1/8" diameter 5A molecular sieve adsorbent pellets.
The above-described patents set forth processes which utilize adsorbents having a particular particle size or particle size range selected to achieve a desired result with respect to either pressure drop or mass transfer limitations. Unfortunately, it has not been possible to obtain the benefits of smaller adsorbent particles, i.e., better mass transfer, in processes wherein low pressure drops are required.
In another patent, i.e., European Patent No. 0,128,998, an alternate approach to improving mass transfer characteristics is disclosed. The above-identified patent discloses a method for improving the dynamic adsorption properties, i.e., mass transfer, of pelleted sodium aluminosilicate zeolites characterized by treating the zeolites with aqueous mineral acid, i.e., HCl. However, this treatment can add complexity and increased costs to the manufacturing process. Moreover, as is known by those skilled in the art, acidic environments can be detrimental to adsorbents and can adversely affect the physical integrity thereof.
Accordingly, processes are sought which can combine the beneficial aspects of smaller particles, i.e., better mass transfer, with the beneficial aspects of larger particles, i.e., lower pressure drop without additional chemical treatments. Such processes are particularly desired in PSA processes wherein regeneration, i.e., desorption, is accomplished at low pressures. Because the regeneration is conducted at low pressure, the regeneration gas has a high volumetric flow rate and, hence, can cause higher pressure drops and reduced regeneration efficiency.